The present invention relates to a gear-shift mechanism for manual transmissions, and more particularly to a gear-shift mechanism of the type which includes first, second and third shift heads arranged in sequence, the first and second shift heads each being operatively connected to first and second synchronizers respectively for establishing first and second speed gear trains and for establishing third and fourth speed gear trains, and the third shift head being operatively connected to a shift fork for establishing a reverse gear train, and further includes a shift-and-select lever operatively connected to a manual shift lever to be axially shifted in selecting operation of the manual shift lever for selective engagement with the shift heads and to be rotated at its shifted position in shifting operation of the manual shift lever for establishment of the selected gear train.
In general, such a conventional gear-shift mechanism as described above does not include any synchronizer for the reverse gear train. For this reason, there will occur unpleasant gear noises in quick shifting operation for establishment of the reverse gear train during slight forward movement of the vehicle. For the purpose of reduction of fuel consumption in recent years, lubricating oil of low viscosity is used to decrease internal resistances of the transmission, and antifriction bearings are adapted to decrease a drag torque in the transmission. For these reasons, when the clutch is disengaged in a condition where the transmission is in its neutral position during arrest of the vehicle, the input shaft of the transmission tends to rotate for a while after disengagement of the clutch. This results in occurrence of unpleasant gear noises in the transmission in its quick reverse shifting operation.
In a manual transmission having a reverse gear train of the constantly meshing type, a synchronizer for the reverse gear train can be adapted in a relatively simple manner to avoid the above-mentioned drawbacks. However, the following disadvantages will remain.
1. During forward movement of the vehicle, the reverse gear is driven by the output shaft and slides on the output shaft at a high speed. For this reason, it is required to enhance anti-sticking property of the reverse gear, resulting in increase of the manufacturing cost of the transmission.
2. In operation of the transmission, the reverse gear is permanently in mesh with the reverse idler gear. This results in increase of drag resistances in the transmission, occurrence of unpleasant gear noises during operation of the transmission in its neutral position, decrease of the torque transmission efficiency, and increase of equivalent moment of inertia in shifting operation for establishment of the forward gear trains.
To avoid such disadvantages as described above, it is desirable to adapt a reverse gear train of the selective slide type to the transmission. It is, however, difficult to install additional members such as a synchronizer ring, a shifting key and the like for a synchronizer in the transmission.